Despite recent advances in targeted and immunotherapies, melanoma remains a deadly disease. Although the majority of melanomas are cutaneous in origin, 5% initiate in the uvea of the eye. Efforts to better understand the genetics of melanoma have resulted in the identification of numerous recurrently mutated genes. In particular, the importance of GTP binding proteins that undergo oncogenic mutations has been increasingly recognized. Mutant NRAS represents one well-known cytoplasmic GTPase in this setting. More recently, a whole exome sequencing project led by our group has identified a novel activating mutation in the small GTPase, RAC1. The specific role of mutant RAC1 in melanoma, however, remains unknown. In uveal melanoma (UM), frequently mutated genes include the heterotrimeric G protein alpha subunits GNAQ and GNA11, in addition to the tumor suppressor BAP1. Mutations in GNAQ/11 lead to downstream activation of the MAP kinase pathway, which signals through RAF, MEK, and ERK. Clinical trials of MEK inhibitors are underway, although responses have thus far proved disappointing, suggesting that alternative therapeutic strategies are necessary for effective treatment of this malignancy. The goal of this research is to characterize cellular effectors and dependencies linked to mutated GTP binding proteins in melanoma biology and therapeutic resistance. To accomplish this goal, two aims seek to (1) define the role of RAC1-dependent signaling in melanoma biology and resistance to MAP kinase pathway inhibition, and (2) systematically identify genes whose silencing sensitizes GNAQ/11-mutant cells to MEK inhibition.